1. Field of the Invention
The present invention relates to a mobile communication system and a radio network controller for transmitting a plurality of same information to a plurality of cells.
2. Description of the Related Art
A mobile communication system for transmitting a plurality of same information (multicast data or broadcast data) to a plurality of cells (or sectors) has been known. FIG. 1 shows the entire configuration of such a mobile communication system.
As shown in FIG. 1, the mobile communication system in the related-art includes a core network CN and a radio access network UTRAN. The radio access network UTRAN includes a plurality of radio network controllers RNC #1 and #2, and a plurality of Node Bs (e.g., base stations) #1a to #2b. 
The core network CN is connected to the radio network controllers RNC by Iu interfaces. The radio network controllers RNC are connected to each other by an Iur interface. The radio network controllers RNC are connected to the Node Bs by Iub interfaces.
A mobile station UE is configured to receive the same information transmitted from one or more Node Bs through a plurality of cells. In the example of FIG. 1, the mobile station UE receives the same information transmitted from the Node Bs #1a and #1b through a plurality of cells.
As handover processes in the related-art mobile communication system, an intra-frequency handover process, an inter-frequency handover process, an inter-radio access technology (RAT) handover process and so on are known. As an example of the inter-RAT handover process, a handover process between a GSM system and a W-CDMA system is known.
In the related-art mobile communication system, each mobile station UE is configured to measure radio channel quality in a cell adjacent to a cell in which the mobile station UE is located, in order to determine whether to perform the intra-frequency handover process or not.
If radio channel quality in the current cell is lower than that in the adjacent cell, the mobile station UE performs the intra-frequency handover process so as to change the radio channel for use in communication with the radio access network UTRAN, from the radio channel in the current cell to the radio channel in the adjacent cell.
In order to perform the inter-frequency handover process or the inter-RAT handover process for each mobile station UE in the related-art communication system, it is necessary that the mobile station UE measures radio channel quality on a frequency which is different from a frequency on which the mobile station UE is currently communicating.
Measurements of radio channel quality on two or more frequencies for the inter-frequency handover process or the inter-RAT handover process are implemented by inter-frequency measurements or inter-RAT measurements.
In the inter-frequency measurements or the inter-RAT measurements, the measurement object is a Primary Common Pilot Channel (CPICH).
Measurement items include downlink Ec/Io, downlink path loss, downlink received signal code power (RSCV), downlink signal-to-interference ratio (SIR) after despreading in a specific physical channel, and average SIR of all downlink codes belonging to a specific Coded Composite Transport Channel (CCTrCH) and so on.
Referring to FIGS. 2 to 4, the inter-frequency handover process or the inter-RAT handover process performed for a mobile station UE in the related-art mobile communication system will be described in detail below.
As shown in FIG. 2, in step S1001, the radio access network UTRAN transmits a measurement control message to the mobile station UE, so as to instruct it to perform inter-frequency measurements or inter-RAT measurements.
The measurement control message includes measurement items to be measured at the mobile station UE and report items to be transmitted to the radio access network UTRAN.
In step S1002, based on the received measurement control message, the mobile station UE performs the inter-frequency measurements or the inter-RAT measurements. Hereinafter, with reference to FIGS. 3 and 4, the operation of the mobile station UE in step S1002 will be described in detail.
As shown in FIG. 3, in step S1101, the mobile station UE starts to operate in compressed mode.
FIG. 4 shows the transmission timing of multicast data by the radio access network UTRAN in compressed mode. Specifically, in compressed mode, the radio access network UTRAN is configured not to transmit multicast data in transmission gaps.
In the example of FIG. 4, the radio access network UTRAN transmits multicast data #1 to #7 to a plurality of cells on a frequency f1 in periods except periods #2 and #7. The mobile station UE is herein performing communication with the radio access network UTRAN on the frequency f1.
In step S1102 (period #1), the mobile station UE monitors a Primary CPICH being transmitted on the frequency f1 from the radio access network UTRAN, so as to perform measurements on the measurement items specified by the measurement control message.
Also, the mobile station UE performs selective combining using a plurality of multicast data #1 transmitted on the frequency f1 from a plurality of cells.
In step S1103 (period #2, transmission gap #1), the mobile station UE stops communication with the radio access network UTRAN, on the frequency f1, and monitors a Primary CPICH being transmitted on a frequency f2 from the radio access network UTRAN, so as to perform measurements on the measurement items specified by the measurement control message.
Herein, the mobile station UE does not have the capability to receive signals of different frequencies simultaneously.
In step S1104 (period #3), the mobile station UE restarts communication with the radio access network UTRAN on the frequency f1, and performs selective combining using multicast data #2 transmitted on the frequency f1 from a plurality of cells.
In step S1003, based on the measurements in step S1002, the mobile station UE generates a measurement report including the report items specified by the measurement control message for transmission to the radio access network UTRAN.
In step S1004, based on the received measurement report, the radio access network UTRAN instructs the mobile station UE to perform the inter-frequency handover process or the inter-RAT handover process.
As a result, the mobile station UE changes the frequency on which to perform communication with the radio access network UTRAN, from the frequency f1 to the frequency f2.
In the related-art mobile communication system, however, in order to perform the inter-frequency handover process or the inter-RAT handover process, it is necessary that the mobile station UE stops communication with the radio access network UTRAN so as to perform the inter-frequency measurements or the inter-RAT measurements.
Also, since different mobile stations UE start to operate in compressed mode at different timings, the radio access network UTRAN cannot make transmission gap timings for all the mobile stations UE coincide.
This causes a problem in that there is a possibility in that the mobile stations UE cannot receive multicast data transmitted from the radio access network UTRAN while stopping communication with the radio access network UTRAN.